This invention relates generally to the field of telephony, and more particularly to an improved solid state protector module for individual subscriber circuits normally engaged with a telephone protector block in a telephone company central office. Devices of this general type are known in the art, and the invention lies in specific constructional details which permit improved ease of manufacture and assembly, and the availability of fail-safe operation in the event of failure of the principal heat-sensitive protector element.
While traditionally, protective modules of the instant type have been equipped with so-called carbon arc devices which serve to short excess current and voltage surges on the protected line to a source of ground potential, because of improved economies in manufacture, such devices have been almost uniformly replaced by units offering either gas tube or solid state protective elements. Generally, gas tubes had been more popular, principally because they are cheaper to manufacture. However, solid state protector devices are particularly useful in protecting circuits connected to solid state office equipment, because of greater sensitivity and faster reaction time.
As with all protector modules, the principal protective element serves to ground momentary current surges of excess voltage, and since such protective devices are destroyed when subjected to substained excess current loads, it is usual to provide a heat-sensitive secondary protective means which responds to heat generated within the module during such sustained current overload.
In the case of solid state protective modules, the momentary surge protector element is usually manufactured as an integrated circuit chip which is encased within a synthetic resinous enclosure having electrically conductive leads projecting therefrom and communicating with tip, ring, and ground connections. During overload, the enclosure will overheat, and in some cases, actually melt. However, the destruction of the integrated circuit chip may occur in such manner that a shorting to ground may not always occur. Where additional heat coil protection is provided, such elements are also subject to failure. There thus arises the need of a fail-safe grounding means operated by the heat generated within the housing of the module which will assure such grounding operation.